pgsql: Fix unintentional 'NULL' string literal in pg_upgrade.

Fix unintentional 'NULL' string literal in pg_upgrade.

Introduced in 2a083ab807.

Note: backport of commit 945126234b, which was missed at the time.

Discussion: https://postgr.es/m/e852442da35b4f31acc600ed98bbee0f12e65e0c.camel@j-davis.com
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/57467ec7b3d3769d6e96c3995ca4060ac0167901

Modified Files
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src/bin/pg_upgrade/pg_upgrade.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)